A bold mission study led by the Finnish Meteorological Institute (FMI) suggests planetary scientists could harness the capabilities of nano-spacecraft to conduct an exploration survey of the asteroid belt on a scale never seen before. The study, presented today at the 2017 European Planetary Science Congress (EPSC) in Riga, Latvia, outlines a plan to send 50 nanoprobes propelled by electric solar wind sails (E-sails) to conduct flybys and science observations at more than 300 of the largest and most interesting asteroids in the belt. After a 3.2-year tour of the asteroid belt, the probes would return to Earth to download the mission data, comprised mainly of surface images and spectroscopy readings.

"Asteroids are very diverse and, to date, we've only seen a small number at close range. To understand them better, we need to study a large number in situ. The only way to do this affordably is by using small spacecraft," says Pekka Janhunen of FMI, who presented the study at the European Planetary Science Congress.

The mission concept, known as the Asteroid Touring Nanosat Fleet, would require the probes to return for a flyby of Earth after the asteroid flybys, as the nano-spacecraft would only be able to carry a small antenna with a relatively short range for transmitting data. The nanosats would fly within 100 kilometers of their target asteroids, resolving the surfaces of the space rocks to 100 meters or better using 4-centimeter telescopes affixed to each probe. In addition to the small imaging telescopes, the probes would each carry a spectrometer to determine the chemical composition of the asteroids. The two instruments on each tiny craft could be pointed at the target asteroids using internal reaction wheels—a flywheel and electric motor combination developed for minute spacecraft movements.

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"The nanosats could gather a great deal of information about the asteroids they encounter during their tour, including the overall size and shape, whether there are craters on the surface or dust, whether there are any moons, and whether the asteroids are primitive bodies or a rubble pile," says Janhunen. "They would also gather data on the chemical composition of surface features, such as whether the spectral signature of water is present."

The propulsion system for the nanoprobes is a unique solar-energy harnessing method of producing thrust developed by Janhunen himself in 2006. An electric solar wind sail is nothing like a traditional solar sail, which is a large reflective surface that is propelled when hit by photons, either from the sun's light or from an artificial source such as a laser.

Diagram of a single-tether E-sail spacecraft.

Pekka Janhunen et al

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An electric solar wind sail, or E-sail, on the other hand, consists of a main spacecraft and a smaller remote unit joined by a electrically charged tether stretching 20 kilometers long. The craft would harness energy from solar wind—electrically charged particles (ions) that are constantly emitted from the sun—to generate a small amount of thrust. An electron emitter powered by the charged particles would release excess electrons to maintain the correct charge in the tether. The entire craft would rotate around the center of mass, close to the main spacecraft, spinning around every 50 minutes to trace out a shallow cone shape. By changing the craft's orientation relative to the sun, the nanoprobe can change thrust and direction.

However, on paper, Janhunen predicts the probes could generate a small amount of thrust using E-sail technology. A 5-kilogram spacecraft with a 20-kilometer tether could accelerate at 1 millimeter per second at the distance of Earth to the sun, according to the mission study. This would allow the craft to complete their asteroid belt tour in just over 3 years.

Janhunen has estimated the total mission cost for the Asteroid Touring Nanosat Fleet at just 60 million Euros (72 million USD), significantly cheaper than the billions of dollars an interplanetary spacecraft typically costs. It's an exciting idea that would make use of a propulsion system that has a myriad of possible applications—from sending a fast spacecraft to the edges of the solar system to using an E-sail as a brake on an interstellar probe propelled by a traditional light sail.

If CubeSat and laboratory tests demonstrate E-sail propulsion is in fact viable, Janhunen might just convince the astronomy community to launch his 50 nanosats on a whirlwind tour of the asteroid belt. It would be an unprecedented opportunity to test dozens of E-sails in a real exploration mission, and the little probes could discover wonders we never dreamed of on the millions of chunks of rock and ice orbiting in our little neighborhood in space.

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